Start with the part that is verifiable
Separate the event you can describe from the motive or reaction you are guessing. For a message about how to apologize for not checking on a friend, use this factual base: identify the moment, the impact on trust or inclusion, and what your friend should not have had to carry.
Make one answerable request
Give the recipient one concrete question or next step instead of several emotional jobs at once. Offer a repair that matches the harm and show how your behavior will be different. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Leave the recipient in control
Do not use shared history, mutual friends, or public gestures to pressure a private response. If contact is unwelcome, stop and avoid using mutual friends as messengers. A clear message remains useful even when the answer is no or no answer arrives.